A precise description of the membrane of each gamete is required before details of their interaction during fertilization can be understood. Qualitative, but no quantitative, data support the concept that the plasma membrane of sperm undergoes continual modification during spermatogenesis, maturation within the epididymis, ejaculation and passage through the female tract. We have developed physical methods to detect changes in the sperm plasma membrane and have establsihed when, during epididymal transit, these changes occur as well as the temporal relation to these changes to the acquisition of motility and fetility. One general conclusion drawn from these data is that an alteration in one membrane feature, fluidity, could account for most of the phenomenological observations. I request support for a three-year period to study sperm membrane structure, with special emphasis on evaluating changes in plasma membrane fluidity and its resultant effect on surface related phenomena. Specifically, I propose to: (a) synthesize spin labels designed to assess fluidity of plasma membranes of intact cells, validate their use, and utilize these compounds to evaluate the changes in fluidity of sperm plasma membranes that occur during epididymal transit; (b) characterize a surface related event, lectin binding, that is controlled in part by membrane fluidity. It is evident that membrane modifications precede the acquisition of fertility. I feel that the proposed quantitative description of the maturation-associated modifications of the plasma membrane of ram sperm will identify critical or vulnerable points in the maturation process and could provide the basis for modulation of epididymal maturation in vivo.